Control knob (4)
I believe the answer is:
boss
I believe this is a double definition.
'control' is the first definition.
(I've seen this before)
'knob' is the second definition.
(I know that knob can be written as boss)
(Other definitions for boss that I've seen before include "One in charge; stud" , "Domineer" , "Team manager" , "Leader; stud" , "Head honcho" .)